Disable FFmpeg logs for encoder

[NicolasHug]

And let the user define the log level through TORCHCODEC_FFMPEG_LOG_LEVEL, like in the decoder.

[scotts]

I think we need <cstring> to get std::strcmp (https://en.cppreference.com/w/cpp/string/byte/strcmp); see the errors in CI. It only sees strcmp from the C headers.

But, I think we can actually just make things simpler and avoid manipulating C-style strings:

int logLevel = AV_LOG_QUIET;
const char* logLevelPtr = std::getenv("TORCHCODEC_FFMPEG_LOG_LEVEL");
if (logLevelPtr != nullPtr) {
  std::string logLevelEnv(logLevelPtr);
  if (logLevelEnv == "QUIET") {
    logLevel = AV_LOG_QUIET;
  }
  else if (logLevelEnv == "PANIC") {
    logLevel = AV_LOG_PANIC;
  }
  ...
}

There's a marginal cost in copying the string, but we execute this code once at startup, and we should prefer C++ string operations over C-style string operations.